[Bug c++/78173] New: Hard error subtracting pointers to incomplete type in SFINAE context

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78173

            Bug ID: 78173
           Summary: Hard error subtracting pointers to incomplete type in
                    SFINAE context
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

Created attachment 39934
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39934&action=edit
Minimal repro

This TU, which should compile without errors:

template<class T>
T&& declval();

template<class...>
using void_t = void;

template<class, class = void>
struct foo { static const bool value = false; };
template<class T>
struct foo<T, void_t<decltype(declval<T>() - declval<T>())>> {
    static const bool value = true;
};

struct A;

#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)

STATIC_ASSERT(!foo<void*>::value); // error: invalid use of 'void'
STATIC_ASSERT(!foo<A*>::value); // error: invalid use of incomplete type
'struct A'

instead produces diagnostics
(http://melpon.org/wandbox/permlink/EwcDiz7ZuqpFXdHZ):

prog.cc: In substitution of 'template<class T> struct foo<T, void_t<decltype
((declval<T>() - declval<T>()))> > [with T = void*]':
prog.cc:22:1:   required from here
prog.cc:10:44: error: invalid use of 'void'
 struct foo<T, void_t<decltype(declval<T>() - declval<T>())>> {
                               ~~~~~~~~~~~~~^~~~~~~~~~~~~~
prog.cc: In substitution of 'template<class T> struct foo<T, void_t<decltype
((declval<T>() - declval<T>()))> > [with T = A*]':
prog.cc:23:1:   required from here
prog.cc:10:44: error: invalid use of incomplete type 'struct A'
prog.cc:14:8: note: forward declaration of 'struct A'
 struct A;
        ^